Certain plating baths, in particular nickel and nickel-iron or other nickel alloy plating baths, will contain as additives organic materials. In many cases organics are complexers or reducers and as part of the reactions in the electrolytic baths, these organic compounds will decompose and/or react. A common decomposition or reaction product is the oxalate ion. The oxalate ion may be formed as a result of direct electrolytic decomposition or even of decomposition resulting from the presence of oxygen (or other oxidizers) formed at the anodes or introduced by air agitation. It is commonly formed in baths containing gluconates, ascorbates, erythorbates, other carbohydrates and the like.
As a result of the formation of the oxalate ion (however formed in the bath) a precipitate of insoluble nickel oxalate has been forming in these baths and, in particular, this precipitate (which may contain other occluded or absorbed materials such as carbonaceous materials or insoluble sulfides) has been forming in and about the anode bag cloth material. Eventually these anode bags become both hardened and clogged with this material so that flow of the electrolytic solution is impeded. When the process has proceeded far enough, it becomes necessary to remove the old bags and replace them with new ones, an expensive operation.
There are, in general, two ways that the insolubilizing cation can be introduced to the plating bath:
1. The cation could be added in the form of a soluble salt, e.g., as calcium sulfate. In such case beneficial actions may be obtained by general precipitation of the oxalate (i.e., calcium oxalate) in a form which does not clog the anode bags even though some is formed therein and by the fact that this other oxalate does not form the hardened matrix layer now formed by nickel oxalate. Of course, the precipitate must not deteriorate the electrodeposit.
2. The cation could be introduced as part of a sparingly soluble salt and preferably before a filter element. In parallel to the case above, calcium fluoride may be added. In these cases locally higher concentrations of the sparingly soluble salt would cause exchange of the anions i.e. the fluoride ion for the oxalate ion and so the general concentration of the oxalate in the body of the electrolyte would be kept low enough to prevent precipitation of nickel oxalate in the anode bags.